1. Field of the Invention
The present invention relates to a television tuner configured to be capable of receiving an FM broadcast signal, in addition to a television signal.
2. Description of the Related Art
FIG. 5 illustrates the construction of the primary portions of a conventional television tuner capable of receiving a television signal and an FM broadcast signal. Either the received television signal or FM broadcast signal is selected and input to a mixer 11, and mixed with a local oscillating signal (not shown) supplied to the mixer 11 to be converted into an intermediate frequency signal in the television intermediate frequency band. The intermediate frequency signal is input to an intermediate frequency amplifier 13 via an intermediate frequency tuning circuit 12.
Here, the intermediate frequency tuning circuit 12 includes a capacitive element 12a, two serially connected inductance elements 12b and 12c, which are connected in parallel to the capacitive element 12a, and a varactor diode 12e connected in parallel to the capacitive element 12a and the inductance elements 12b and 12c via a DC-cut capacitive element 12d. Also, the power supply voltage Vcc supplied to the connection point of the two inductance elements 12b and 12c, is supplied to the mixer 11. The intermediate frequency tuning circuit 12 and the intermediate frequency amplifier 13 are connected to each other by the DC-cut capacitive elements 14 and 15.
The cathode of the varactor diode 12e is supplied with the power supply voltage and the anode thereof is connected to a switch circuit 16. The switch circuit 16 is switched via a resistor such that the anode of the varactor diode 12e is either grounded or connected to the power supply to be supplied with a voltage Ve lower than the power supply voltage.
In the above-mentioned construction, upon receiving the television signal, the television signal of the selected channel is input to the mixer 11. Also, the anode of the varactor diode 12e is grounded via the resistor by the switch circuit 16. Therefore, the voltage between the both ends of the varactor diode 12e becomes the power supply voltage Vcc. By the capacitance value of the varactor diode 12e, the tuning frequency of the intermediate frequency circuit 12 is set to be positioned between a video intermediate frequency P (58.75 MHz) and an audio intermediate frequency S (54.25 MHz), as illustrated in FIG. 6.
Further, upon receiving the FM broadcast signal, the FM broadcast signal of the selected frequency is input to the mixer 11. Moreover, the anode of the varactor diode 12e is supplied with the voltage Ve by the switch circuit 16. Therefore, since the voltage between the both ends of the varactor diode 12e decreases to voltage Vcc−Ve, the capacitance value of the varactor diode 12e increases. By the capacitance value of the varactor diode 12e, the tuning frequency of the intermediate frequency circuit 12 is set to be positioned at an audio intermediate frequency S (54.25 MHz), as illustrated in FIG. 7 (For example, refer to Japanese unexamined Patent Publication No. 2001-157128 (FIG. 1, pages 3 to 4)).
Since the FM broadcast signal has a narrow frequency interval (200 kHz) with an adjacent broadcast signal, the selectivity of the intermediate frequency tuning circuit must be sharp in order to avoid interference therebetween. However, since the intermediate frequency amplifier is tightly connected to the intermediate frequency tuning circuit by the DC-cut capacitive element, and the input impedance of the intermediate frequency amplifier is low, there is the problem in that the intermediate circuit is Q-damped and thus the selectivity cannot be sufficiently sharp.